Document U.S. Pat. No. 5,083,911 describes a high pressure casting method for producing sanitaryware.
The diagram of FIG. 1 shows a typical example of a time/pressure graph of a pressure casting mould. More specifically, the method according to prior art comprises the steps described below.
The parts of the mould, or moulds in the case of a machine with two or more moulds, are closed and clamped to each other.
In other terms, there is a mould closing step: the way this is done and the time taken depend on the type of installation (casting bench) and on the number of moulds installed.
During this step, the pressure is not varied and remains at the level of atmospheric pressure.
The mould closing step is followed by a step of cleaning the supply pipe leading to the mould with a new ceramic mixture. The mixture will hereinafter be referred to as “slip”.
The slip is made to circulate inside the main supply pipe so as to remove any impurities and condition the pipe itself before the mould is filled with the slip.
Next, the discharge valve is closed and the mould is filled with the new slip. At the same time the slip also enters a pressurising device connected to the slip supply pipe.
During this step, there is an increase in the pressure, represented by the first section 1-1a in the time/pressure graph.
This is followed by a first step of forming the thickness of the sanitaryware article. In this step, the walls of the article develop thickness as the pressure is increased to a predetermined level and for a length of time corresponding to section 1a-2 of the graph of FIG. 1.
The pressure applied to the slip by the pressurising device forces the water in the slip out through the mould pores and is drained out through appropriate passages formed in the mould. These passages may be connected to a vacuum source such as to facilitate the drainage of the water out of the mould by the resulting suction effect.
This creates a layer of ceramic material that is deposited and adheres to the inside walls of the mould to start forming the shape of the sanitaryware article.
This is followed by a second step of forming the thickness of the sanitaryware article at a constant pressure for a length of time corresponding to section 2-3 of the graph of FIG. 1.
When the ceramic material has formed a layer of predetermined thickness inside the mould, the pressurising device is switched off, this step corresponding to section 3-4 of the graph of FIG. 1.
Next, the discharge valve is opened and a compressed air valve is also opened so as to blow a jet of compressed air through the article in order to force the excess slip out through a drainage pipe.
This step is represented by sections 4-5-6-7 in the graph of FIG. 1 which show an intermediate increase in the pressure caused by the counterpressure created by the discharging of the excess slip.
Next, once all the excess slip has been discharged, the discharge valve is closed and compressed air is injected so as to pressurise the article again.
Next, there is a first step of consolidating the thickness of the sanitaryware article by increasing the pressure of the compressed air for a length of time corresponding to section 7-8 of the graph FIG. 1.
This is followed by a second step of consolidating the thickness of the sanitaryware article by injection of compressed air at a constant pressure for a length of time corresponding to section 8-9 of the graph FIG. 1.
The article is then subjected to a decompression step: the air pressure is allowed to fall from the maximum consolidation value to a value substantially equal to atmospheric pressure, corresponding to section 9-10 of the graph of FIG. 1.
The last steps in the process are the opening of the mould—or the moulds in the case of a machine with two or more moulds—and the demoulding of the article inside.
The times and methods of performing the opening and demoulding steps depend on the type of installation (casting bench) and on the number of moulds installed, as for the mould closing step at the start of the casting method described here.
The casting method according to prior art includes several steps in which the sanitaryware article is subjected to sudden pressure variations where the pressure is increased or decreased. The pulsation caused by these pressure changes on the mould and article being formed may lead to problems.
The efficiency of the resin mould may be seriously reduced mainly on account of cracking of the mould itself or clogging of the open pores that control the filtration process.
The continual stress caused by the pressure pulsation thus leads to damage not only to the moulds themselves but also to the articles cast in the moulds.
Another disadvantage connected directly with the stress on the sanitaryware articles is the restriction it poses on the design of the articles themselves, which means that their geometry and form (thickness, curvature radius, passage from free to constrained thickness).
Moreover, the maximum casting pressure cannot exceed certain limits so as to avoid creating excessive strain and demanding fatigue cycles on the moulds and sanitaryware.
This also means that the casting rate cannot be further increased and the productivity of the casting machine therefore remains relatively low.